The primary components of a modulated carrier wave communications system are (1) modulation of a carrier frequency signal with a data signal, (2) transmission of the modulated signal from one location, (3) reception of the modulated signal at a second location, and (4) demodulation and recovery of the data signal. This basic system must be modified when secure and covert communications are required.
When secure communications (difficult for others monitoring the communications to determine the encrypted data signal) are required, further encoding (typically of pseudo-noise) is also accomplished. In order to decode a pseudo-noise encoded signal, the recipient must have the same encoded pseudo-noise to subtract it from the received signal. The pseudo-noise (signals that appear to have noise-like properties) is combined with the data signal, typically by an "exclusive or" operation. This spreads the spectrum of the modulated signal from a narrow carrier band to a wide (noise-like) spectrum. The embedded carrier signal must not be easily distinguishable from the pseudo-noise signal to prevent its unauthorized capture. The transmitted signal (combination of pseudo-noise and data signal) must appear to be noise to others monitoring the communications. The decoding process essentially lines up by phase comparison or synchronization the received noise-like signal with the pseudo-noise coding, revealing the original data signal as differences between signal and pseudo-noise code "words".
For military applications, secure transmissions must be able to meet other requirements, such as covertness while making it difficult for others to recognize the transmission as communications and to decode it. Although absolute protection against interception and decoding is not possible, a low probability of intercepting (LPI) and decoding is desired. This has required periodic alteration of pseudo-noise code "words" so that its repetition does not reveal it. The possibility of either continuous or periodic jamming, with broad spectrum or narrow band signals must also be considered.
These military requirements have resulted in pulsed transmissions or "burst" transmissions (to minimize time to detect or jam) with variable pseudo-noise code "words". The receiver detects when pulsed signals are being transmitted, turns on, determines the end of the complete transmission, determines the currently valid code "word", and synchronizes the valid code "word" with the received signal to extract the data signal. This process typically takes a few seconds to several minutes to accomplish.
These military processes and communications equipment also have other objectives. The equipment should be small so that it may be camouflaged, not be an obvious target or interfere with other operations. It should also be light weight, rugged in construction, easily operated and low in cost. When the equipment is used in each of the two modes (transmitting and receiving), a minimum of effort to convert from one mode to another mode is also desirable.
Current military or other LPI/secure communications equipment and methods may accomplish some of these objectives well, but others poorly. The signal detection, pulsing, variable encoding and decoding equipment tends to be complex and cumbersome, limiting transport and use. However, the most serious limitations are primarily related to the need to transmit, receive and synchronize a complete signal over the wide (spread) spectrum of pseudo noise frequencies. The transmitter and receiver must be able to perform well over this wide range of spread frequencies as the received signal must be synchronized against a copy of the complete pseudo-noise signal to reveal the data signal. Data can be lost due to poor receiver response time, cross signal interference, enemy jamming, and delays due to synchronization requirements. Some of these limitations can be partially overcome with redundant signal transmissions (multiple pulses with different pseudo-noise "words" producing a different spectrum of frequencies), but this again requires variation of code "words" and the attendant increase in transmitter and receiver system complexity and unreliability.
The greatest limitation of current LPI technology relates to this inherent lack of covertness. All use some kind of periodic transmission with random phase shifts or changes in frequency to achieve the wide band width characteristics of noise. But the periodic transmissions are relatively easy to intercept using correlation type receivers called chip rate detectors. These receivers detect the periodic chip rate characteristic of current military signals.
What is needed is a secure LPI communications device and method that will reliably communicate with less than a complete signal transmission being received, avoid periodic signals and have the characteristics of random noise including a wide bandwidth and a noise-like waveform. It should also be capable of code word synchronization within a fraction of a second.